Serena Williams has made a career of doing the unprecedented, the unfathomable, the indomitable. So it will be no surprise if her announcement Wednesday that she is expecting her first child in early fall has left Williams poised to add another epic chapter to her career. There are a few nagging little reasons to wonder if she can crush this latest challenge, too, and resume being the greatest tennis player who ever lived, if she wants to. But it's all up to Williams. And that's always been more than enough before.

The fact that Williams is having a baby is not, taken alone, reason to hedge on what's possible if she plans to return in 2018. Plenty of other female athletes have come back from having children to notch career milestones even within the first year that they're back.

Even if she decides to make a comeback closing in on 37 years old, there's no reason to doubt Serena Williams won't be dominant again. Jason Heidrich/Icon Sportswire

What is fascinating and different about Williams' case is that none of those other women, amazing as they were, battled the confluence of challenges that Williams will have to overcome. That's what makes this twist in her story so tantalizing.

Normal rules and limits don't apply to Williams, as anyone who's been paying attention already knows. And still, after she posted a photo of herself on Snapchat Wednesday with the cryptic caption "20 weeks," social media was on fire with expressions of awe anyway. Many people quickly realized what William's math meant, that she won the two-week-long Australian Open when she was eight-to-nine weeks pregnant. Are you kidding?

Once again, she was even more amazing than anyone knew.

And so, while it's easy to predict what might get in Williams' way if she comes back, there's no reason to say those hurdles will be enough to stop her from winning more Grand Slams or being No. 1 again.

Nothing has so far derailed her in her historic two-decade career.

The most obvious challenges will be her advanced age, the nature of the sport she plays and whether her ennui about having to hit the road again -- something she was already fighting even before this news -- becomes even worse when she's married and experiencing motherhood for the first time.

Nannies are great. FaceTime chats are wonderful. But at some point plenty of athletes just decide they've had enough and they want to go home. Williams herself played only eight events in 2016. For years now, she's skipped the last four months of the season after the US Open and year-end WTA Championship.

After Serena Williams posted a photo of herself standing sideways with the caption "20 weeks" on Snapchat, a spokeswoman confirmed the 23-time major champion is expecting a baby this fall.

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"You never know how someone will react to something like this," said ESPN analyst Pam Shriver, who had her three children after she retired from tennis. "Becoming a mother might cause Serena to say 'Game, set, match' and call it a career. Or she could look at her time away as an opportunity, the same way Roger Federer approached his break last year, and view it as a chance to rejuvenate her body and mind after so many years on tour. That's a challenge that sounds right up her alley to me."

Williams, who turns 36 in September, is already the oldest woman ever to win a Grand Slam singles title. She's still the most skilled and physically gifted player on the women's tour. She still displays the titanic will that's revealed itself in many well-known examples (screaming at a US Open lineswoman over a shockingly timed foot fault call in the 2009 semis) and not-so-famous stories.

When Williams was a young player, there was a tale making the rounds about how she appeared to be sleeping with her head resting on her crossed arms on a conference table during a very long meeting with some sneaker company executives who were balking at paying her more endorsement money. They didn't know she'd heard every word until she startled them by abruptly snapping up her head and saying, "WHAT? You don't think I'm worth it?"

But how long can even Williams outrun time?

Foot speed and movement have always defined Williams' game, along with her serve.

She owns an Open era record of 23 Slam titles. But she'll be closing in on 37 by next summer, leaving her anywhere from three to 10 years older during her comeback than other high-achieving moms who notched milestone wins.

British heptathlete Jessica Ennis-Hill was 28 when she won gold at the 2015 IAAF World Championships nine months after returning to training following the birth of her son, Reggie.

British marathoner Paula Radcliffe resumed running just 12 days after giving birth to her first child, Isla, and won the 2007 New York Marathon 10 months later at age 33. But Radcliffe, who spent 27 hours in labor, battled injuries and now says she wouldn't recommend returning so soon.

It was big news when Belgian tennis star Kim Clijsters had a daughter, Jada, in 2007 and then entered the 2009 US Open as a wild card and beat Williams in the semis on the way to winning the title. The feat made Clijsters the first mother to win a Grand Slam singles crown since 29-year-old Evonne Goolagong Cawley won the 1980 Wimbledon championship three years after giving birth to her first daughter.

Clijsters was only 26 when she returned.

But Williams can take heart in this: There are two well-known examples of athletes who were older than she will be next year. Golfer Catriona Matthews was 40 when she became the first Scotswoman to win the British Open in 2009, just 11 weeks after having her second child. American swimmer Dara Torres was 41 when she won silver medals in the 50-meter freestyle and two 4x100 relay events at the 2008 Summer Olympics just 16 months after giving birth to a daughter.

Williams can certainly add her name to that list. The gap between her talent and the rest of the world is far more significant than the margin even Torres, one of the most decorated female swimmers of all time, ever enjoyed.

If Serena Williams comes back, she would be looking to join only three other tennis players who have won a Grand Slam after having a child. EPA/PAUL BUCK

And still, as much as Williams' fans hate to hear this, there have been more and more occasions in the past couple of years where Williams has not lost matches because of her own errors or indifference -- she was beaten fair and square. At the 2016 US Open, even her coach, Patrick Mouratoglou, decried her form in her finals loss to Angelique Kerber, saying Williams had a down year.

Now look: Williams will have been out of action 16 months, dating back to her Jan. 27 final at the 2016 Australian, if she gives birth by early September and hustles back in time to play the 2018 French Open next May.

That's a long time to be sidelined in a grueling year-round sport that's based on stamina, foot speed, timing the ball and fast-twitch reactions. A nine-month turnaround isn't a long time to be back in training. Tennis is a one-on-one game. There are no teammates on court to rely on when you have an off day. Everything's on you.

But again, if anyone is uniquely qualified to handle it all, it's Williams. Even more than her older sister, Venus, who is still chugging along at 36, Serena has been able to take long breaks from the tour and come back as good or better than ever.

So no, motherhood needn't mean the end of a historic run for Serena Williams. It could be quite the opposite. Her maternity leave may be nothing more than an interlude before the start of another history-making run that's even more impressive than anything anyone has done. She's already the best tennis player ever. Now she has a chance to be best ever in the gloaming of a career, too.